1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device which is capable of securing desired transistor characteristics and realizing the class AB bias.
2. Description of Related Art
There is a case where a class AB biasing method is adopted in an amplifier requiring high linearity. In the case of a high frequency input signal (for example, 2.4 GHz), the class AB bias can be realized, for example, as follows. An alternating current (hereinafter referred to as AC) signal supplied through a capacitive coupling and a direct current (hereinafter referred to as DC) bias signal supplied through a resistor are superimposed on each other, so as to be inputted into the gate of a transistor whose source is grounded to a reference potential, such as GND. Here, desired characteristics of the amplifier (transistor) can be obtained by generating a DC bias by using, for example, a current mirror circuit biased by a current source that maintains constant transconductance (gm). With this configuration, when the input amplitude of the AC signal inputted into the gate of the transistor is increased, the DC output current of the transistor is also increased, so that the amplifier can be class AB biased (see, for example, D. Yamazaki, et al., “2.5-GHz fully integrated WiMAX transceiver IC for compact low-power-consumption RF module”, IEEE Radio Frequency Integrated Circuits Symposium, pp. 109-pp. 112, and 2008, (pp. 110, FIG. 2)).
However, there is a problem that in the same configuration, when the input signal is a low frequency signal (for example, 5 MHz), and when the AC signal is to be capacitively coupled by a capacitor, a large capacitance is needed, thereby increasing the circuit scale. Thus, when in order to realize the class AB bias of the amplifier by eliminating the capacitive coupling, the output signal of the preceding stage circuit (=the preceding stage output common mode DC voltage+the preceding stage AC output signal) is directly inputted into the gate of the transistor, the characteristics (such as gm) of the transistor are determined by the common mode voltage of the preceding stage circuit, and hence it is difficult to obtain desired characteristics (such as gm). Here, as a method for obtaining the desired characteristics of the transistor, a method can be considered in which a level shifter is inserted into the preceding stage of the transistor so as to obtain a desired DC level (VDC). However, there newly arises a problem that the distortion is caused and noise characteristics are inevitably deteriorated by inserting the extra circuit. When a configuration is adopted, as another method, in which the current flowing into the source of the transistor is regulated by a constant current source in order to obtain the desired characteristics, the transistor becomes class A biased, so that the linearity is deteriorated.